This invention relates to the field of laminar sheet material. More particularly, this invention relates to the field of laminar circuit sheet material comprised of a fluoropolymer/glass composite layer sandwiched between a polyimide film and a copper foil.
Conventional flexible circuitry comprised of polyimide film such as DuPont's Kapton film adhesively bonded to thin metal foil, usually copper, is well known to those skilled in the art. Typically, a conductor pattern is printed on the copper with an etch resistant coating, the unnecessary copper subsequently being etched away. While finding great application, this conventional flex circuitry suffers from certain drawbacks and deficiencies. For example, substrate materials having lower dielectric constants will offer less interference to electronic signals traveling through the conductors especially at high frequency. The dielectric constant of conventionally used polyimide substrate film such as Kapton, is typically 3.5 (dielectric constant of Kapton is dependent on humidity). With the increasing use of high speed signals, the relatively high dielectric constant of polyimide substrates create undesirable effects. Consequently, a substrate material having a lower dielectric constant, i.e., less than about 2.5, would be highly desirable from at least the standpoint of offering less resistance to electronic signals.
Another common problem with polyimide based flex circuitry is found in the necessary adhesive associated therewith. Adhesives between the polyimide film and copper conductors may be adversely affected by high temperature.
One way of overcoming the above discussed resistance problem is to cover the polyimide film with a material having a relatively lower dielectric constant and which does not need an adhesive bond. Fluoropolymers are materials which satisfy these two requirements. In fact, laminar structures of polyimide and fluorocarbon polymers have been described in U.S. Pat. Nos. 3,676,566 and 3,770,566 assigned to E. I. duPont de Nemours and Company. Commercially, polyimide/fluoropolymer laminates are well known and available from the E. I. duPont de Nemours and Company under the trademarks Kapton F and Kapton XP.
Unfortunately, while fluoropolymer coated polyimides (such as Kapton F or Kapton XP) provide an improved, i.e., lower, dielectric constant than conventional polyimide (Kapton) film coated with an acrylic or epoxy adhesive, other serious problems associated with fluoropolymers are presented. The two most important problems involve poor dimensional stability of the fluoropolymer and poor adhesion or bond strength between the copper conductors and the fluoropolymer film.
In an effort to overcome these problems, U.S. patent application Ser. No. 629,164, now abandoned and U.S. patent application Ser. No. 755,067, (which is a continuation-in-part of U.S. Ser. No. 629,164 and is being filed contemporaneously with the present application), both of which are assigned to the assignee hereof and incorporated herein by reference, provide a laminar circuit sheet material wherein a layer of microglass reinforced fluoropolymer is sandwiched between a fluoropolymer coated polyimide laminate, i.e., Kapton F or Kapton XP, and a copper conductive pattern. In essence, the fluoropolymer/microglass film of the present invention acts as a bonding layer or adhesive which greatly improves the bond strength between, for example, the Kapton F or XP and the etched copper conductors.
Moreover, use of microglass reinforced fluoropolymer film not only improves the bond between, for example, Kapton F or XP and copper foil, but also, and just as significantly, improves the overall dimensional stability of the laminate. This improved dimensional stability is of particular importance, since, as mentioned, fluoropolymers (and even polyimide) suffer from poor and at times, unacceptable dimensional stability. Furthermore, the microglass reinforced fluoropolymer bonding layer also provides improved temperature characteristics to the laminated circuit sheet of the present invention.
Thus, the laminate circuit material disclosed in U.S. patent application Ser. No. 629,164 results in a lower dielectric constant relative to conventional polyimide (Kapton) film based flexible circuit material due to the presence of the fluoropolymer. Also, the dimensional stability and bond strength to copper foil in the laminate of the instant invention are improved relative to fluoropolymer coated polyimides such as Kapton F or XP.
However, while suitable for its intended purposes, the laminate circuit material disclosed in U.S. Ser. No. 629,164 suffers from several deficiencies and disadvantages including, but not limited to the following:
(1) The laminate circuit material of U.S. Ser. No. 629,164 is relatively thick due to the presence of both fluoropolymer and fluoropolymer adhesive layers.
(2) The layer(s) of fluoropolymer film provide decreased dimensional stability.
(3) The cost of fluoropolymer coated polyimides such as Kapton F or XP is quite expensive.
(4) The overall quality of the laminate product may be difficult to control.